Please note:

If you've ever done filet crochet, this should be fairly easy. (And if you have, I would recommend working from the second chart, since that's more filet crochet-ish.)

I put a round of USsc/UKdc alternating with ch1 around the entire thing: one USsc into every other stitch on the top and bottom, and two USsc into each turning chain on the sides, with one USsc around each ending USdc.

Instructions

In pictures

The above is an after-the-fact chart. My working chart looked more like this: